Downhole valves are used to isolate portions of the wellbore for a variety of reasons such as for safety systems or to allow building a long bottom hole assembly in the wellbore, to name a few examples. Such valves have featured a rotating ball with a bore through it that can be aligned or misaligned with the path through the tubing string where the valve is mounted. The ball is surrounded by a sliding cage that is operated by a hydraulic control system from the surface. One such design that features opposed pistons actuated by discrete control lines is illustrated in US Publication 2009/0184278. This design was concerned about a pressure imbalance on an operating piston and provided a passage through the piston with two check valves 64, 70 in series to allow pressure equalization across the actuating piston with the ball in the closed position.
What can happen in this type of a ball valve that has upper and lower seats against the ball in the closed position is that pressure from downhole can rise, which leads to a pressure differential between the passage inside the ball and the downhole pressure. This pressure differential can distort the ball and make it hard or impossible for the piston actuation system to operate the ball back into the open position. One way this was solved is described in a commonly assigned application Ser. No. 12/366,752 filed on Feb. 6, 2009 and having the title Pressure Equalization Device for Downhole Tools. The solution described in this application was to use a tool that goes into the upper sleeve that hold a seat against the ball and separate the seat from the ball while providing pressure from the surface at the same time to equalize the pressure on the ball before trying to rotate it to the open position. The problem with this technique was that it required a run into the well with coiled tubing, latching and shifting the upper sleeve and associated seat enough to give access into the ball for equalizing pressure. One of the downsides of this technique was that the pressure admitted to try to equalize the pressure in the ball could be high enough to unseat the lower seat from the ball so that the higher pressure below the ball would get to above the ball. This technique also took time which cost the operator money and required specialized equipment at the well location, which could be remote or offshore and add yet additional costs to the effort to operate the ball when subjected to high differential pressures that increases opening friction or could distort the ball enough to make it hard for the hydraulic system to rotate it.
In flapper type safety valves such as U.S. Pat. No. 5,564,502 the preferred method to get pressure equalization on a closed flapper was to simply apply tubing pressure on top of it to reduce the differential before using the control system to try to rotate the flapper. Of course, the flapper is built to rotate open with pressure applied above so that this technique did not equalize pressure around the flapper when it was closed but simply built up pressure above it when it was closed. Other equalizer valves mounted in the flapper were actuated by the hydraulic system moving down a flow tube that impacted the equalizing valve before the flapper was engaged by the flow tube as seen in U.S. Pat. Nos. 6,848,509 or 4,478,286.
Also relevant are US Publications 2001/0045285; 2009/0184278 and U.S. Pat. Nos. 4,130,166; 4,197,879; 4,288,165; 4,446,922; 5,865,246; 6,223,824; 6,708,946; 6,695,286 and 4,368,871.
The basic components of the valve of FIG. 1 are reviewed in more detail in US Publication 2008/0110632 whose description is fully incorporated by reference herein as though full set forth. The portions of such valve relevant to the understanding of the present invention will be reviewed below in sufficient detail and for completeness so as to fully understand the operation of the claimed invention. While the actuation system of the valve in FIG. 1 in the present case is somewhat different in that it uses mechanically operated rod pistons to move the ball cage, the remainder of the structure of the ball and the way it seals and turns are the same with the further exception that the present invention is employed to equalize pressure as between the inside of the closed ball and the pressure below the ball by virtue of application of uphole pressure to accomplish a bypass of an uphole seal to achieve pressure equalization.
Those skilled in the art will better understand how pressure equalization is obtained before the ball is turned from a review of the detailed description of the preferred embodiment and the associated drawings while recognizing that the technique is by no means limited to downhole ball valves but can be used in a variety of tools where trapped pressure results in differentials that may damage the component to be moved or the actuating system for it if such differentials are not resolved before attempting to move the component. Those skilled in the art will further understand that the full scope of the invention is to be found in the appended claims.